Hepatitis C virus (HCV) infection and HIV coinfection act synergistically to promote hepatic fibrosis and cirrhosis, but the underlying pathogenic mechanisms remain unclear due to a lack of appropriate animal models. The BalbC rag2/?C double knockout (DKO) mouse lacks T, B and NK cells and supports development of a functional human immune system in all lymphoid organs. To promote human liver cell co-engraftment, we introduced a liver-specific inducible suicidal transgene (AFC8) in the DKO mouse. Co-transplantation of human blood and liver progenitor cells in AFC8 mice leads to development of both human liver cells and immune cells in lymphoid/liver organs (AFC8-hu mice). These mice are permissive for HCV as well as HIV infection, support anti-viral human T cell responses, and develop human liver immunopathology (inflammation, hepatitis and fibrosis). I postulate that HIV co-infection may affect HCV-induced liver diseases by enhancing HCV replication; by elevating chronic hepatic inflammation; and by activating hepatic stellate cells to accelerate liver fibrosis. The AFC8-hu model is well suited for elucidating the mechanism underlying HCV/HIV synergy in human hepatic fibrosis in vivo. We propose the following aims: 1) The current AFC8-hu mouse will be improved to support higher human hepatocytes in the chimeric liver. In addition, HCV gt 1a and 2a clones will be tested to establish their infection and pathogenesis kinetics. 2) To study how HIV co-infection exacerbates HCV-induced liver fibrosis in AFC8-hu mice. We will define how HIV infection influences HCV infection, immunopathogenesis, stellate cell activation and liver fibrosis. In addition, the effectof HCV infection on HIV-1 replication and AIDS progression will also be monitored. 3) I propose that HIV coinfection dysregulates human inflammatory and immune responses to contribute to HCV-associated liver diseases. We will define the role of key HIV target cells pDC and Treg that promote HCV-induced liver stellate cell activation and fibrosis in vitro and in vivo. The novel AFC8-hu mouse is unique in providing a small animal model in which these important questions can be addressed. The answers to these questions will have a very significant impact on the field.